This invention relates generally to doped gallium arsenide semiconductor devices and particularly relates to such a device having epitaxially grown buffer and active layers with very low background doping.
It has previously been suggested to grow an eptiaxial gallium arsenide layer on a gallium arsenide substrate having relatively low background doping. In this process it was necessary to utilize arsenic trichloride (AsCl.sub.3. The lowest doping obtainable with a quartz tube using arsine (AsH.sub.3) and (HCl) hydrogen chloride was 4 .times. 10.sup.14 doping of active carrier atoms per cubic centimeter. If a quartz tube is utilized incorporating a boron nitride liner, the background doping level can be reduced to 1.9 .times. 10.sup.14 atoms per cubic centimeter.
There are several disadvantages to the known process. In the first place boron nitride liners are very expensive and may in turn cause additional problems. Furthermore a doping level of 1.9 .times. 10.sup.14 atoms/cm.sup.3 is not useful for FET's. Additionally, arsenic trichloride cannot be adapted to large scale production. The reason is that in the formation of GaCl.sub.3, GaAs formed over the Ga melt limits the amount of GaCl.sub.3 formed. The reaction somewhat simplified is as follows: EQU 3H.sub.2 + 2 Ga + 2 AsCl.sub.3 .fwdarw.2 GaAs + 6 HCl (1)
One problem with this reaction is that a skin of gallium arsenide forms on the gallium in the reservoir of the furnace. This skin slows down the reaction rate and hence limits the production available with this system.
The prior art is exemplified by a paper by John W. Burd which appears in Transactions of the Metallurgical Society of AIME, Vol. 245, March 1968, pages 571 - 576. Among other things this paper discloses a furnace suitable for the type of reaction to which the present invention relates.
It is accordingly an object of the present invention to provide an improved process for growing epitaxially gallium arsenide on a doped gallium arsenid substrate in such a manner that the epitaxial layer has very low background doping.
Another object of the present invention is to carry out the process in a furnace having a quartz liner which tends to provide silicon doping while maintaining the dopant at low levels.
A further object of the present invention is to provide a reaction of the type disclosed wherein hydrogen chloride is supplied at a level sufficiently low to substantially prevent etching of the epitaxial layer but sufficiently high to substantially inhibit doping of the epitaxial layer with silicon.
Still another object of the present invention is to provide a process for epitaxially forming a layer of gallium arsenide having a first conductivity on a substrate of gallium arsenide having a second conductivity in an enclosure having an inner liner comprised of a silicon compound, comprising decomposing arsine to form arsenic; chemically reacting a first quantity of hydrogen chloride and gallium to form gallium chloride, the gallium chloride reacting with the arsenic to form gallium arsenide on the substrate, a portion of the first quantity of hydrogen chloride remaining unreacted and tending to cause silicon contaminants from the liner to be deposited on the substrate; and providing a second quantity of hydrogen chloride into the enclosure which serves to initially etch, and hence clean, the outer surface of the substrate prior to the formation of gallium arsenide thereon, and simultaneously tends to inhibit the formation of silicon contaminants on the substrate, the second quantity and the first quantity having a preselected ratio such that the growth rate of the gallium arsenide layer on the substrate is greater than the etching rate of gallium arsenide due to the second quantity of hydrogen.